The present invention relates to a thermal transfer printer, and more particularly to a thermal transfer printer employing an ink ribbon coated with a thermal melting ink thereon or a heat sensitive paper.
In a conventional thermal transfer printer employing an ink ribbon, when a roughened surface paper such as a bond paper is used therein, the printing quality of the thermal transfer printer is undesirable because of an unevenness of ink transfer.
The inventors of the present invention have found an inconvenience in that the ink, which desirably is at the central portion of the printing dot, is missing when the face pressure of a thermal head against a platen is increased for improvement of the unevenness of ink transfer.
A conventional thermal head will be explained with reference to FIGS. 13-17. The thermal head 100 having a heating resistor element mounted on a projected top portion of a graze portion is disclosed in, for example, Japanese Patent Laid-Open No. 159365/1984. The conventional thermal head 100 comprises a head substrate member 101 and a graze portion 102 mounted on the surface of the head substrate member 101 as shown in FIG. 13.
The graze portion 102 is conventionally made of ceramics material and has a heating resistor element 103 at the top central portion or crown of a circular cross-sectionally shaped projection on the graze portion 102. The projection on graze portion 102 forms originally a shape illustrated by the curved broken line 104; however, the the graze portion 102 except for the projection containing the heating resistor element 103 has part of its surface removed as by an etching processing.
By this procedure the heating resistor element 103 is made to extend further from the surface of the graze portion 102. The horizontal width Wo of the heating resistor element 103 in the conventional thermal head 100 is made to be about 180 .mu.m.
When the thermal head 100 having the above stated shape and installed in a printer of the type shown in FIG. 1, is pressed against a platen 19, the face pressure distribution of the graze portion 102 of the thermal head 100 is shown by line P.sub.o in FIG. 14.
All regions of the heating resistor element 103 of the graze portion 102 of the thermal head 100 have a high face pressure in comparison with the face pressure of the other portions of the thermal head 100. The raised portion of the heating resistor element 103 of the graze portion 102 is extended for the purpose of reducing printing unevenness. In FIG. 14, the broken line T.sub.o shows the temperature distribution of the heating resistor element 103 of the graze portion 102 of the thermal head 100.
The face pressure distribution P.sub.o of the heating resistor element 103 of the thermal head 100 has a region of increased pressure at both ends thereof as shown in FIG. 14 when compared with the central region thereof. The face pressure distribution P.sub.o of the heating resistor element 103 of the thermal head 100 extends substantially uniformly over all regions thereof, in comparison with the face pressure of the graze portion 102 except for the portion of the heating resistor element 103.
The temperature distribution of the heating resistor element 103 of the conventional thermal head 100 is shown in FIGS. 15 and 16 when the thermal head 100 is not pressed against the platen.
The curve line T.sub.01 shown in FIG. 16 indicates the temperature distribution of the heating resistor element 103 of the thermal head 100. The temperature distribution of the heating resistor element 103 of the thermal head 100 shows a maximum temperature at the central portion and a temperature profile which indicates a lower temperature at opposite ends of the heating resistor element 103.
The contour lines RL.sub.o shown in FIG. 15 indicate isothermal curve lines of the heating resistor element 103 of the thermal head 100. The heating resistor element 103 of the thermal head 100 has the horizontal width W.sub.o and the lengthwise width L.sub.o, respectively.
When printing is practiced by the thermal head 100 employing the above stated temperature distribution T.sub.o and the face pressure distribution P.sub.o, ink is omitted from the central portion of the printing dot. Consequently, the printing dot D.sub.o becomes a doughnut-like dot as shown in FIG. 17 having an ink void in the center portion.
In the conventional thermal transfer printer as shown in FIG. 1, the face pressure distribution P.sub.o is substantially uniform over the region of the heating resistor element 103 in the thermal head 100. Also, the heat resistance of the heat transfer system extending over the heating resistor element 103, the ink ribbon 9 and the transfer recording paper 17 is substantially uniform. The central portion of the heating resistor element 103 of the thermal head 100 has a somewhat higher temperature indicated by FIGS. 15 and 16.
Because the central portion of the heating resistor element 103 of the thermal head 100 reaches a high temperature state, the ink at the central portion is melted but not transferred to the recording paper 17. Instead the central portion is cooled and allowed to solidify. The ink not at the central portion is transferred from the ink ribbon side to the transfer recording paper side and transferred onto the transfer recording paper 17.
In a the conventional thermal transfer printer, the ink of the central portion of the printing dot does not mark and the image has an unmarked hollow center as illustrated in FIG. 17. When the roughened surface paper 17 such as a bond paper is used, the face pressure of the graze portion of the thermal head is increased because of an unevenness of the transfer recording paper surface and the melted ink transfers unevenly into the paper making control of the printing density difficult. To obtain an increase in printing density, it has been necessary to increase the amount of ink required, thus requiring a concomitant increase in heat energy for the thermal head.
As stated above, in the conventional thermal transfer printer, there is a disadvantage in that the central portion of the printing dot remains in a white or ink void condition which detracts from the quality of the print.